In wire electrical discharge machining, insulating machining-fluid is put between a wire electrode and a workpiece as electrodes, and while the wire electrode and the workpiece are moved relative to one another, machining electrical power is supplied to the electrodes by a machining electrical power supply device, and the workpiece is machined by electrical discharge energy.
An example of this type of machining electrical power supply device in a conventional wire electrical discharge machine that uses wire electrical discharge machining is disclosed in Japanese Laid-Open Patent Publication 1996-118147. This machining electrical power supply device is provided with an electrical shorting distinguishing circuit for distinguishing an electrical shorting state between the electrodes, and, if an electrical shorting state is detected, it eliminates the electrical shorting state by applying to the electrodes an electrical current pulse with the minimum peak value necessary for eliminating the electrical shorting, to improve machining speed by avoiding breakage of the wire electrode.
With the machining electrical power supply device in this conventional wire electrical discharge machine, electrical current pulse peak values are set for each of normal, abnormal, and electrical shorting states, but the electrical current pulse waveform cannot be controlled according to the result of the distinguishing circuit. That is, the peak value of the current pulse supplied to the electrodes differs in accordance with the normal, the abnormal, and the electrical shorting inter-electrode conditions, but the waveform has a roughly similar shape.
FIG. 14 illustrates an example of the inter-electrode current waveform for the machining electrical power supply device in the conventional wire electrical discharge machine, and in the figure, V represents inter-electrode voltage, I represents inter-electrode current, and t represents time. Corresponding to the waveform of the inter-electrode voltage V of FIG. 14(a), the inter-electrode current I has roughly similarly shaped waveforms, as in FIGS. 14(b) and (c).
With the machining electrical power supply device in the conventional wire electrical discharge machine, even in the high-speed machining field, in which machining gaps are narrow, an energy change in the current pulse flowing to the electrodes in order to eliminate the electrical shorting merely controls changes in the peak values, as described above, and cannot perform fine energy control. When machining speed is increased in the high-speed machining field close to the breaking limit of the wire electrode, the controllable area for avoiding the breakage of the wire electrode becomes narrow, and thus the breakage limit of the wire electrode is easily exceeded, and since it is difficult to safely avoid the electrode wire breakage, ultimately there has been a problem in that the machining speed cannot be increased.
Further, in order to perform high-speed machining, the electrical load per current pulse must be made large in order to make the electrical discharge energy large; however, the higher the peak value of the inter-electrode current the more easily the wire electrode is broken, and with the conventional inter-electrode current waveform as in FIG. 14, in order to make the electrical discharge energy large, the area of the inter-electrode current waveform is made large, and the only way to do this is to raise the peak value; thus, there has been a problem in that it is not possible to improve the machining speed while inhibiting the breakage of the wire electrode.
In addition, because the frequency of electrical shorting increases in the high-speed machining field, the current pulse, which flows to the electrodes in order to eliminate electrical shorting, contributes to bridge elimination in electrical shorting members but not directly to the electrical discharge itself, and thus, in the high-speed machining field, there has been a problem in that electrical power that does not directly contribute to the machining increases.